Questions and Answers

The questions (with accompanying answers) on this page, are in reply to some
of the requests for information that we have received, and to clarify some
commonly held misconceptions about the minor bodies of the solar system
and space debris.

(Please note that due to staff and time limitations, we are generally unable
to answer individual queries directly, but we will attempt to reply on this
page if we feel your query is of general interest. Technical queries are more
likely to be dealt with here than others.)

Q1 What is the difference between a meteor, a meteorite and a meteoroid?

ANSWER - A meteoroid is the name given to any small object moving in the solar
system. The upper size for a meteoroid is not well defined, but is
often regarded to be around 10 metres. Meteoroids may be chunks of
stone or iron (in which case they probably originated from asteroids),
or they may be much lower density friable material (in which case they
probably originated from a comet). Meteoroids are more common the
smaller they are. That is, meteoroids the size of a grain of sand are
much more common than meteoroids 1 metre across. Even more common are
meteoroids only a few microns (millionths of metre) across.

If and when a meteoroid passes close enough to the Earth to be captured
by the Earth's gravity, it will enter the upper atmosphere, and a
process of ablation will occur. In this process, the kinetic energy of
the meteoroid is reduced by collision with the air molecules, heating
up the outer layers of the meteoroid and causing them to vapourise and
fragment. This process is visible to observers on the Earth as a
meteor. At the same time as the light and heat are produced, the air is
ionised (electrons are stripped from the air molecules), and this
ionisation may be detected by a meteor radar.

It is most important to understand that the term meteor refers to the
visible (or radio) phenomenon that the meteoroid produces in our upper
atmosphere. The word meteor should not be used to refer to the
physical body (i.e. a meteor is not the name for the meteoroid when it
is in the atmosphere) In fact, the general definition of the term
meteor is any phenomenon that occurs in the atmosphere - this was
coined originally by meteorologists, but is rarely used in this general
context any more - unless prefixed, as in the term hydrometeor.

Most of the time, the meteor process reduces the meteoroid to no more
that a scattering of dust, and this slowly drifts down through the
atmosphere over a period of a few years. Occasionally however, if the
meteoroid is large enough, a remnant body will be left over when all
the kinetic energy of the original meteoroid has been dissipated by the
atmosphere. In this case, the remains of the meteoroid will fall to
the Earth at terminal velocity (determined by the balance between the
gravitational force and the air resistance forces acting on the mass),
which is typically about 100 metres/second. When this reaches the
surface of the Earth it is termed a meteorite.

So in summary, meteoroid is the name of a small body in space. Meteor
is the term used to describe the phenomenon of the passage of a
meteoroid through the atmosphere, and meteorite is the name given to
any fragment of a meteoroid that reaches the surface of the Earth.

Q2 How does one find out what bodies are on a collision course with the Earth?

ANSWER - As of the end of 2003, the Minor Planet Center listed over 500 asteroids
with the designation PHA or Potentially Hazardous Asteroids. This does
not mean however, that any of these asteroids are on a collision course
with the Earth. It does however, indicate both that these bodies could
possibly collide with the Earth in the distant future, and that if they
did so, considerable damage to life on Earth might occur.

Again, as of the end of 2003, no signficantly sized asteroid or comet
was known to be on a collision course with the Earth in the
immediate future. It is not possible to predict orbits of NEOs
accurately for more than a few hundred years into the future. This is
because of two factors. The first is due to the limited accuracy of
the initial observations, and the second is due to perturbations or
changes in the orbits of NEOs produced by other bodies in the solar
system.

If you want to find out the current situation with regard to potential
Earth impacts by known asteroids or comets, there are two web sites,
one in the USA, and the other in Italy, which keep updated pages on any
known NEO hazards. These are:
JPL: neo.jpl.nasa.gov/ca/
- lists NEO close approaches and
neo.jpl.nasa.gov/risk/
- lists NEO impact risks
NeoDys Risk Page:
NeoDys

Q3 Why was this project named Wormwood?

ANSWER - The name was chosen from an ancient text as purely descriptive. It has
no significance beyond that, and no deeper meaning should be read into
the name. The staff of the project have no special scientific insights
into any future NEO orbital behaviour! There is also no classified
activity associated with the project.

Q4 What damage is likely to be caused by the impact of an NEO on the
Earth?

ANSWER - This depends enormously on the size of the object, where on the Earth it
hits, and what it is made of.

A cometary fragment about 100m across will generally not make it to the
ground, but will produce an "airburst", equivalent to a large nuclear
bomb detonated in the atmosphere. An asteroidal fragment of the same
size will generally make it through the atmosphere, and if it impacts
on land produce a crater at least one kilometre in diameter. In both
cases the energy released is enough to destroy a large city, but only if the
incident occurs near a large city. However, it is much more probable
that such an impact will occur over the ocean with no noticeable
effects, or if over land, it will be well away from large
population centres.

A body one kilometre across, if impacting the ocean, may create a
damaging tidal wave. If on land, it is likely to be much more
damaging, laying waste a region the size of a large State.

A body ten kilometres across will undoubtedly create global effects,
raising devastating tsunamis if in the ocean, or laying waste whole
countries if on land. Consequent effects, such as vast dust clouds
lofted into the atmosphere, could make conditions for most life around
the world very difficult for several years on.

More detail on specific effects are provided in the information section
of this web page and the references contained therein. Check also our
web links section.

Q5 What should governments be doing to prepare for and protect their
citizens against NEO impacts?

ANSWER - NEO impacts of significant size are extremely rare (a rough estimate
is that a 10km object, which has a global impact, might be expected
about every 100 million years). An impact of a body such as this would
overwhelm any conceiveable civil defence plan that any government could
put in place. Because of this it is generally conceded that the only
possible defence is to try and stop such an impact in the first place.

Following the realisation that an impact threat does exist (something
which we have only come to widely appreciate in the last two decades),
the next step is to try and locate all potentially hazardous objects
before they impact, and finally to try and take steps to avert the
impact through space based means. As awareness of the potential impact
threat has grown so has the activity to deal with it. Governments,
universities, corporations and private individuals around the world
have all become involved. Conferences dealing with these issues have
been held regularly since around 1990.

For the many smaller sized bodies, which have a greater chance of
escaping the observation network, but which also carry a much smaller
damage bill, probably the best defence that any government can do is to
ensure that the country has the best possible emergency service
networks to deal with all the other natural (and man-made) hazards to
which we are subject on this Earth.

The Australian Government has a plan in place to deal with any serious
emergency that might affect a large number of people. This would apply
equally to a populated area suffering from a small hypervelocity impact
as it would to a city (such as Darwin) that had been devastated by a
tropical cyclone. The plan is maintained by
Emergency Management Australia
and is one of four emergency management
plans. This one is called COMDISPLAN, short for Commonwealth
Government Disaster Response Plan.

Q6 Is it possible for an individual to assist in Planetary Defence?

ANSWER - Yes, this is certainly possible. Many amateur astronomers have equipment
that can detect the brighter asteroids and comets, and the ability to
submit very useful reports to the Minor Planet Center. If you are
unable to individually purchase such equipment, you might consider
joining one of the many amateur astronomical societies in Australia and
around the world. Even if your local astronomical society does not
currently have a program dedicated to searching and performing
follow-up observations on NEOs, you may be able to lobby them to do so.
The first step is to read all you can about NEOs and astronomical
measurement techniques in general (and there is no dearth of
information of this kind around at present). Use the links from this
web site as a starting point.

There is another opportunity you may like to consider, and this
involves only a high speed connection to the internet. The Spacewatch
Project of the Lunar and Planetary Laboratory of the University of
Arizona has now set up a project in which anyone can join in the
detection of fast moving objects (FMOs) from the images that they
acquire. You do need a fast internet connection (the images involved
are very large), a good eye, and an appropriate amount of time. For
more information go to
Spacewatch-FMO.

Several dedicated amateurs also peruse data from the SOHO satellite
instrument LASCO (a solar white light coronagraph) to find Sun grazing
comets. In fact, through their activities, SOHO is now credited with
more comet discoveries than any other single individual, team or
program. The LASCO web site is located at
lasco-www.nrl.navy.mil.

NASA also has a program called
Skymorph,
by which individuals
have discovered comets using only a Personal Computer and the internet.

And, if you read the answer to the last question, you will realise that
by contributing your time and effort to any emergency service
organisation (such as volunteer fire brigade, ambulance, state
emergency service or rescue organisation), you are also helping prepare
your country (and yourself) for any possible future Planetary Defence
scenario.

Q7 Are any asteroids visible to the naked eye?

ANSWER - Most asteroids are very small, and they orbit the Sun between the orbits
of Mars and Jupiter, which puts them a long way from the Earth.
Because of these two factors, almost no asteroids are routinely visible
from the Earth without optical aid (eg a telescope). Even the largest
known main belt asteroid, Ceres, at 900 km in diameter is always
invisible to our eyes. However, there is one exception, and that is
Vesta. Vesta is not among the four largest asteroids, but it is the
most highly reflective asteroid known, and at times of opposition (when
it is closest to the Earth), it just becomes a naked eye object. Not
that it is easy to see. You will need a dark sky location well away
from city lights, and a good star atlas, to help you distinguish it
from the many magnitude 6 stars around. Sometimes the popular
astronomy magazines run a feature on observing Vesta when it is at
opposition.

There is also a very very slight chance that once in a while, a Near
Earth Asteroid might pass close enough to be able to observe visually.
However, NEA's are all much smaller than the large main belt asteroids,
and they appear to move very rapidly when close to Earth, so that you
would probably be very lucky to see a single NEA within your lifetime.
And you will probably have to be closely watching all the MPC
(Minor Planet Center) and JPL (Jet Propulsion Lab)
web sites to learn of such an observing opportunity.

Q8 When are comets visible to the naked eye?

ANSWER - At any one time there are probably several dozen comets that are visible
in a large telescope. However, only a very few of these will ever
become bright enough to be visible to the naked eye. The brightness of
a comet depends both on its distance from the Sun and its distance from
the Earth. If these were the only two factors involved then a comet's
brightness could be predicted as accurately as that of an asteroid.
However, it is the third factor, cometary activity, which also affects
the brightness, and as this is unpredictable, so we find that
predictions of a comet's brightness are also unreliable.

Sometimes a comet will be observed at a very great distance from the
Sun, and astronomers may then predict that this comet should become
very bright, to the point where it will be visible to the naked eye, as
it approaches perihelion (the closest point in its orbit to the Sun),
and as it passes near the Earth. Generally a comet becomes more
active, throwing off gas and dust, as it gets closer to the Sun, and
the Sun's heat causes sublimation (turning from a solid to a gas) of
the volatile material of which comets are composed. This materials
forms the well known tail behind the comet. It also determines the
comet's reflectivity and thus brightness.

In one year it may be possible to see two or three comets with the
naked eye. There may then be a stretch of several years in which no
comets are visible. Because comets are generally small objects (no
more than a few tens of kilometres across) they cannot be seen far from
the Sun where they are inactive in the coldness of space, and so it is
not possible to predict years ahead when a naked comet will be visible.

Then again, there are some exceptions to this rule. Some comets have
well known periodic orbits (such as Halley's comet) of small enough
period that it is possible to predict their return with good accuracy.
What it is never possible to predict is exactly how bright they will be
when they return. This again is because of the variability in their
activity.

Q9 What is the chance of being hit by a meteorite or a piece of space
debris?

ANSWER - Very low indeed! In fact, books about meteors up until the internet age
used to quote time after time that "no one has ever been killed by a
meteorite". The internet has now put historians in contact with
meteorite researchers, and most will agree that throughout recorded
history, there probably are several cases where people have in fact
been killed by meteorite strikes.

In the last decade, there are also a few recorded instances of damage
to people and property, although no deaths. A car has been dented, and
one holed, a house roof has been breached, and a person hit, amongst
others. In 2003, the town of Park Field in Illinois suffered multiple
damages from a meteorite "shower" (probably fragments from the one
meteoroid that entered the Earth's atmosphere moments before).

It should be noted that the above incidents are from meteorites that
are travelling relatively slowly (about 100 metres per second). They
have been decelerated by the Earth's atmosphere from the initial
meteoroid speed of maybe 20 kilometres per second. They form no crater
when they hit the ground.

Despite the above records, you are much much more likely to be killed
in a car accident than to be non-lethally struck by a meteorite. And
if you do happen to have such luck, with the current fetching price of
meteorites, you can probably reap enough financially to pay any
hospital bills with some left over (except in states such as Western
Australia where all meteorites have been declared the property of the
State).

The probability of being hit by a piece of man-made re-entering space
debris (satellite fragment) is even lower. While we know that a few
pieces of space debris make it to the ground each year (without being
totally burnt up in the atmosphere - which most are), there are no
recorded incidents of anyone being hit by such an object, despite some
of these pieces being recovered every now and then by enthusiastic
prospectors.

Q10 Does the Australian Government have a plan in place to deal with
re-entering space debris?

ANSWER - Satellites in low Earth orbit are subject to orbital decay due to
atmospheric drag. After a time such satellites may re-enter the lower
levels of the Earth's atmosphere where they will be mostly consumed;
burnt up due to the high temperatures encountered in the re-entry
process. Occasionally very large pieces or very refractory structures
may not be totally detroyed, and they may fall to the ground.

The Australian Government has set up a plan to deal with such
re-entering space debris that is termed AUSCONPLAN-SPRED, the
Australian Contingency Plan for Space Re-entry Debris. This is
maintained by Emergency Management
Australia. If any
of the debris is known or suspected to contain radioactive material
(and in fact this is really the only kind of debris that is worth
worrying about), then the Government agency ARPANSA (Australian
Radiation Protection and Nuclear Safety Agency) will be involved
www.arpansa.gov.au/arlspace.htm.

Q11 Where can one find a list of all asteroids?

ANSWER - The
Minor Planet Center
maintains lists of asteroids. These are broken up into sub-lists
which contain specific types of asteroids (eg potentially harzardous
asteroids, named asteroids in alphabetical order, etc).
If you want to download the complete orbital
information for all named and numbered asteroids, be prepared
for a very large file!